1. Field of the Invention
This invention relates to a programmable controller of a stored program type, and more particularly to a programmable controller capable of prohibiting an external output at an optional step or release it to continue the sequential operation from the state before the prohibition.
2. Description of the Prior Art
FIG. 1 is a schematic block diagram of the programmable controller of stored program type, in which a CPU (central processing unit) 1 serving as the control nucleus operates according to the contents stored in a system memory 2, reads a program related to the sequential operation in succession from a program memory 3 storing therein the program, and decides it. In a case where an instruction defined by the program refers to the state of input-output signal, CPU has accesses to an input/output memory 4 to read the necessary information to execute necessary computations in accordance with the instruction, the input/output memory 4 being connected to an input interface 6 and an output interface 7 through an input/output controller 5 to thereby store the state of a switch 8, sensors, etc., connected to the input interface 6. The input interface 6 converts the signal level obtained from the switch 8 and the like into the level necessary for read-in through the input/output controller 5. When the instruction is to store the result of computation, the result, such as output information or the like is written into the input/output memory 4. After such process, CPU gives a command to the input/output controller 5 and reads successively the output information stored in the input/output memory 4 in order to provide the corresponding output interface 7 with a control signal for actuating the external apparatus, such as switch 9, connected to the output interface 7. The output interface 7 holds the control signal until new signals are given thereto and carries out the level conversion necessary to actuate the external apparatus, such as relay 9.
The input interface 6 and output interface 7, which are so constructed that many external apparatuses can be connected thereto respectively, complete one cycle of operation upon finishing the aforesaid input-output operation about all terminals, thus repeating the execution of the cycle operation.
Now, in a case where apparatuses controlled by such programmable controller are subjected to adjustment, inspection and repair, it has been impossible for the conventional programmable controller to temporarily stop the actuating of the apparatuses for adjustment or the like on the way of operation and thereafter continue the predetermined sequential operation from the state before the stop. Presumably, for the temporary stop of sequential operation of apparatuses, the operation of the programmable controller itself needs to be stopped and, when stopped, returned to the initial state, whereby the apparatuses, when they restart, should execute the sequential operation from the first step, which is largely wasteful of time. Especially in a continuous process control including heat treatment, much time elapses before the apparatus returns to the state before the stop, thereby causing a problem in that materials under the heat treatment may be useless.
In order to solve the above problem, the following constructions are well known:
1. A relay is interposed between the output interface of programmable controller and the apparatuses so that the contact of relay is disconnected to cut off the control signal output from the programmable controller.
2. A self-holding circuit is provided at respective circuit for connecting external apparatus of the output interface of programmable controller to thereby disable send-out of output signal (the self-holding circuit, even when the output signal disappears, acts to keep the previous condition).
3. The external output prohibition terminal is provided at the input interface of the programmable controller, which, when given a temporary stop command signal, prohibits the output in one lot.
The above constructions of items (1) and (2), however, require the provision of a relay or a self-holding circuit outside or inside the programmable controller so that the whole construction is complicated, thereby having the defect that a load on the capacity of program memory unit increases and it is difficult to grasp the operation of circuit. In item (3), there is a defect that the collective prohibit of output is not suitable for the continuous process control necessary to hold the operation of apparatuses in part.
In the light of the above problem, this invention has been designed. An object thereof is to provide a programmable controller which temporarily stops operation of apparatuses to be controlled and thereafter enable the predetermined sequential operation to be continued from the state before the stop of operation.
Another object of the invention is to provide a programmable controller which, when applied to the continuous process control including heat treatment, is capable of restarting the process without wasting time at the aforesaid temporary cessation and avoiding a waste of an object under heat treatment.
Still another object of the invention is to provide a programmable controller which is adapted to enable an output only of necessary part to be prohibited, in other words, that of unnecessary part not to be prohibited, to thereby avoid inconvenience according to nonoperation of apparatuses to be controlled, and which keep safety.
The above and further objects and novel features of the invention will more fully be apparent from the following detailed description with the accompanying drawings.